1. Field of the Invention
This invention relates to an electrophotographic process, and more particularly to such a process for forming an image using a photosensitive plate having a number of openings.
2. Description of the Prior Art
As the typical conventional electrophotography, a direct process such as for example electrofax and an indirect process such as xerography are presented. In the former direct process, use is made of a specifically treated recording material coated with a photoconductive material such as zinc oxide. Consequently, there is a drawback in the image contrast as the image formed on the recording material lacks brightness. Moreover, due to the specific treatment, the recording material is heavier than conventional paper and has a different feed from the usual paper. According to the latter indirect process, a high contrast and high quality image is obtained as it uses conventional paper as the recording material to form an image. However, in this indirect process, when a toner image is transferred to the recording material, the recording material contacts with the surface of the photosensitive member and cleaning means are required to make contact with the surface of the photosensitive member when the remaining toner is cleaned, so that the photosensitive member deteriorates each time the transfer and the cleaning is applied. Therefore, the useful life of an expensive photosensitive member becomes shortened which results in a high cost for forming an image.
The improvements for removing said drawbacks of the conventional processes were disclosed in, for example, U.S. Pat. Nos. 3,220,324; 3,680,954 and 3,645,614. In these patents, a photosensitive member of a screen type or a grid type is used which has a number of fine openings. The electrostatic latent image is formed on a recording material by modifying ion flow through the screen or grid, and thereafter the latent image formed on the recording material is visualized. There is no necessity to develop and clean the screen or grid which corresponds to the photosensitive member, so that the useful life of the screen or grid is prolonged.
In U.S. Pat. No. 3,220,324, a conductive screen coated with a photoconductive material is used, and an image exposure is made simultaneously with corona ion flow from the corona discharger so that ions are applied to a recording material through said screen. The corona ion flow is modified by the screen and an electrostatic latent image is formed on the recording member. In this process, the charging of the screen and the image exposure are simultaneously made and it is difficult to charge the photoconductive material to a sufficiently high potential. Therefore the exposure efficiency of an image is deteriorated and it is difficult to obtain a high quality copied image. Further, at the dark portion of the image where ions are passed, when the potential of the screen is raised too high, the applied corona ion is repulsed so that the corona ion is directed toward the light portion in the vicinity of the dark portion of the screen and the good image is difficult to obtain.
According to U.S. Pat. No. 3,680,954, a conductive grid coated with a photoconductive material and a conductive controlling grid are used, an electrostatic latent image is formed on the grid in the image form, and the different electric fields are formed on the grid and the controlling grid so as to modify the corona ion flow to form an image on a recording member. In this process, however, it is quite difficult to hold the controlling grid and the photoconductor grid to provide proper spacing therebetween over a large area. Moreover, the controlling grid absorbs corona which would otherwise be applied to the recording member, so that the efficiency becomes deteriorated. In case of an attempt at forming a positive image, corona on flow having a polarity opposite to the polarity of the latent image is applied, and almost all of this ion flow directs to the latent image to negate the latent image, so that it is difficult to reproduce the positive image.
In U.S. Pat. No. 3,645,614, the screen comprises an insulating material overlaid with a conductive material, wherein the insulating material comprises a photoconductive material. An electric field preventing the passing of ion flow is formed at the openings corresponding to the electrostatic latent image formed on the screen. In this process, it is a drawback that an image obtainable on the recording member is the image reversal of the latent image on the screen.